MONDAY, Dec. 15 (HealthDay News) -- Letting teenssleep a little more by starting the school day a bit later may lower their odds for car crash injury or death, a new study finds.

The researchers found a 16.5% drop in auto accident rates for teen drivers when local high schools moved the start of classes from 7:30 a.m. to 8:30 a.m.

The possible reason? More sleep, more alert driving, the researchers said.

After puberty, adolescents are biologically programmed to stay up about an hour later each night, explained Fred Danner, the University of Kentucky psychologist who co-authored the study. This shift in their biological clocks then conflicts with having to get up earlier to go to high school than they did when they were in middle school, he added.

"It's as if they are operating on West Coast time in an East Coast world," Danner said. People blame teenagers' sleep deprivation on computers and staying up late to e-mail friends, he added. "But there is evidence they get phase-shifted by at least an hour. So you've got biology pushing you later and then you've got the school systems starting an hour earlier. By the end of the week, [kids] are a wreck and our study shows they might actually be in one."

In the study, the researchers surveyed around 10,000 Kentucky students from grades 6 through 12 on their sleep habits and daytime functioning, including auto mishaps. The surveys were completed twice -- first in 1998, when school started at 7:30 a.m., and then again in 1999, when the start time had been moved to 8:30 a.m.

Besides the 16.5% drop in car crashes, the researchers also found that the number of students who got at least eight hours of sleep per night rose from 35.7% in 1998 to 50% after the later school time came into effect.

The study appears in the Dec. 15 issue of the Journal of Clinical Sleep Medicine.

The average teenager probably needs at least eight hours and probably closer to nine hours of sleep, Danner said. And as little as an hour less sleep on school nights can have a cumulative effect. That means that by the end of the week, teens are as impaired as if they had stayed up for 24 hours straight, Danner explained.

Fatigued drivers cause about 100,000 accidents a year and over half of those drivers are 16 to 25 years old, according to the National Sleep Foundation. One 2006 survey by the foundation revealed that 28% of high school students fall asleep at school and 51% have driven while drowsy. Another recent study showed that sleep deprivation also leads to safety problems for college students. A survey of 262 students at the University of North Texas found that 17% of them reported falling asleep while driving.

While there are statistical limitations in the University of Kentucky study, "if you sleep longer and you are less sleepy you are less likely to have a wreck. It simply stands to reason," said Dr. Francisco Perez-Guerra, former director of the Scott & White Sleep Disorders Center at Texas A&M University.

"We have been talking about later morning starts for children for years," he added. "This is not a brand new thought."

However, there are practical and political obstacles to overcome for school systems to change the school day schedule for high school students. "If it could be done, it should be done. The question is, can it be done?" he said.